I wouldn't do it. A q6600 eats a good amount of power and a 6850 is decent enough with its consumption, but personally I wouldn't run it on less than a quality 500w psu. It may run fine, but for how long is the better question.

It should be barely enough if my experience with a 375W PSU, Core i5 2500k, and GTX 550Ti is any indication. Expect a loud fan and very warm PSU exhaust if you load the GPU and CPU. HINT: Poorer efficiency due to 80+ standard

__________________"The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src]

I'd say saving money on power supply isn't the greatest idea ever. It's one thing that can potentially damage anything in your computer. It's too bad power supply is being overlooked. I agree with everyone here, go with minimum 500W and recommend 600W. It would save you money in the long run.